Miniature electric motor having bearing unit

Abstract: A miniature electric motor has a rotor (34) arranged within a casing (31) within which a stator (32) is mounted. A rotary shaft (35) of the rotor is supported by bearing units (36, 37) mounted on spaced ends of the casing. One (37) of the bearing units is provided with a thrust receiving member (51) for supporting an end portion (57) of the rotary shaft in a thrust direction, and a bearing (53) for supporting the rotary shaft in a radial direction within a rotary shaft insertion hole (52). The thrust receiving member has a dimension which is equal to or smaller than an inner diameter of a closed recess (39) formed in the casing and is laid on a substantially planar base surface (54) of the recess. The bearing is securely fixed within the recess in abutment against the thrust receiving member. The rotary shaft insertion hole (52) is in fluid communication with a through-hole (56) formed through the base (55) of said recess. Thus, it is possible to shorten the axial length of the motor and to stably support the rotary shaft whilst preventing deformation of the thrust receiving member. it is also possible to ventilate air when the rotary shaft is inserted into the insertion hole of the bearing impregnated with lubricating oil.

Description: BACKGROUND OF THE INVENTION The present invention relates to a miniature electric motor and more particularly to a miniature electric motor used for audio visual equipment such as CD (Compact disk) players, optical precision equipment such as compact cameras, and office automation equipment such as copiers. Miniature electric motors have been widely used in various fields in addition to the above-specified equipment and have been developed particularly in miniaturization in weight and size with more performance. In a conventional miniature motor, a recess for receiving one of bearing units is formed in a cover member, and an arcuate groove is formed around a bottom of the recess. An air vent hole is formed in the groove. Through-holes are formed in parallel with a rotary shaft in a radial bearing for radially supporting the rotary shaft in addition to a rotary shaft insertion hole. A plate-like thrust bearing member for supporting the rotary shaft in a thrust direction is inserted under pressure into an interior of the recess so as to keep a predetermined gap relative to the radial bearing. In such a motor, it is necessary to mold the arcuate groove in the recess and it is difficult to perform the molding work. In particular, in the case where the cover member is made of metal, a damage of molds for molding and pressing the arcuate groove would be remarkable. As a result, the molds are inferior in durability and a considerable maintenance is required. Because the thrust bearing member is inserted under pressure into the recess, if it is made of synthetic resin in the form of a plate with a good lubricant property, a warp would be likely to be formed in its peripheral portion. Also in the case where the warp would not occur, its central portion is projected to lift away from a bottom of the recess. If the thrust bearing member is deformed in this manner, a gap is generated between the bottom of the recess and the thrust bearing member. Then, the rotor is moved in the axial direction in the motor drive operation so that the contact between the commutator and brushes would be unstable, thus generating change in rotation. In order to prevent the above-described deformation of the thrust bearing member, it is proposed to set the thrust bearing member on the bottom of the recess by decreasing a diameter of the thrust bearing member without any pressure. However, in this case, when the rotary shaft is rotated, the thrust bearing member is rotated together therewith. A service life of the thrust bearing member would be shortened, resulting from the friction between the thrust bearing member and the bottom of the recess. On the other hand, if the bearing would be in contact with the thrust bearing member in order to shorten the overall length of the motor in the axial direction, the rotary shaft insertion hole is clogged with the thrust bearing member. Then, when the rotary shaft is inserted into the rotary shaft insertion hole in assembling the motor, air stagnant in the insertion hole would pass through the interior of the bearing. As a result, the lubricating oil impregnated in the bearing would be splashed in the interior of the motor from the surface of the bearing together with the air. The splashed oil would be adhered to the commutator and the brushes, resulting in failure of the electric contact. The frictional worn powder would be adhered and accumulated thereto to cause the electric conduction between the commutator elements. Accordingly, it is necessary to form the gap between the bearing and the thrust bearing member in the conventional structure. In addition, in the conventional structure, since the arcuate groove is formed, a projection length of a projecting portion where the recess is formed is large. As a result, the overall length of the casing in the axial direction is elongated to make it difficult to thin the motor. JP-A-59153455 describes a miniature electric motor in which a rotor is arranged in a casing within which a stator is mounted, a rotary shaft of said rotor being rotatably supported by spaced bearing units mounted on said casing, wherein at least one of said bearing units comprises: a thrust receiving member for supporting an end portion of said rotary shaft in a thrust direction, said thrust receiving member being laid on a substantially planar base surface of a recess formed in the casing, and a bearing securely fixed in said recess and in pressing contact with said thrust receiving member, said bearing having a rotary shaft insertion hole in which said rotary shaft is received. It is an object of the present invention to provide a miniature electric motor in which the problems described above are reduced. According to the present invention, a miniature electric motor as defined above is characterised in that said rotary shaft insertion hole is arranged to be in fluid communication with the outside of the casing by way of a through hole formed through a base of said recess, and in that said rotary shaft insertion hole and said through hole are maintained in fluid communication around and/or through said thrust receiving member, said thrust receiving member having a size and peripheral shape to enable said fluid communication. An embodiment of the present invention has the advantage that the bearing units support the rotary shaft with stability in the thrust direction without deforming a thrust bearing member and whilst shortening the axial length of the motor. In an embodiment, the rotary shaft is inserted into the rotary shaft insertion hole of a bearing impregnated with a lubricating oil, the air entrained in the insertion hole is removed so as not to splash the oil, retained in the bearing, towards the interior of the motor, and this thereby prevents the oil from adhering to a commutator and brushes and thus enhances the quality of the motor. An embodiment of the invention has the advantage that lubrication of a sliding part between the rotary shaft and the bearing is kept under a good condition to suppress a frictional wear. In an embodiment of the invention, a miniature motor is provided in which the durability of molds for press moulding a casing may be enhanced. In an embodiment, the rotor is rotatably supported by one and the other of bearing units each mounted on each end portion of the casing. The one of the bearing units comprises a thrust receiving member for supporting an end portion of the rotary shaft in a thrust direction, and a bearing securely fixed in the bottomed recess and brought into pressing contact with the thrust receiving member for supporting the rotary shaft in a rotary shaft insertion hole in a radial direction. The thrust receiving member is laid on a flat bottom surface of a bottomed recess formed in the casing, with a dimension equal to or smaller than that of the bottomed recess. The rotary shaft insertion hole and a through-hole formed in a bottom plate of the bottomed recess are in fluid communication with each other. The thrust receiving member may have a predetermined shape to cover a part of the bottom surface, while always maintaining a communication passage between the rotary shaft insertion hole and the through-hole. Preferably, the receiving member has a plurality of apex portions for always communicating the rotary shaft insertion hole and the through-hole through a space between the thrust receiving member and an inner circumferential surface of the bottomed access. Assuming that a centre of the circle of the through-hole is represented by b, in an embodiment the following relation is given: f > e where e is the dimension of the thrust receiving member extending along a diameter of the through-hole and in a direction perpendicular to a line d passing through the centre b and the centre of the thrust receiving member, and f is the diameter of the through-hole. In an embodiment, the rotary shaft insertion hole and the through-hole are always kept under fluid communication with each other through the space, if a circle of the rotary shaft insertion hole is larger than an inscribed circle of the thrust receiving member. A bevelled portion may be formed in an inner circumferential edge of the rotary shaft insertion hole on the side of the thrust receiving member, a bevel circle of the bevelled portion is larger than an inscribed circle of the thrust receiving member, and the rotary shaft insertion hole and the through-hole are always kept under a fluid communication with each other through the space, if a circle of the rotary shaft insertion hole is smaller than the inscribed circle. Preferably, a slant angle  of the bevelled portion relative to a surface of the thrust receiving member is made small so that a sliding area between the bearing and the rotary shaft is enlarged. The thrust receiving member may be substantially in the form of a polygonal shape such as a regular triangular shape, a square, a hexagonal shape or the like. The thrust receiving member may be substantially in the form of a regular hexagonal shape, and the through-hole may be formed in the vicinity of the inner circumferential surface. In one embodiment, the thrust receiving member has a shape similar to a regular square, an arcuate portion having the same centre as that of the inner circumferential surface is formed in an associated apex of the thrust receiving member, and the through-hole is formed into an oblong hole extending in the circumferential direction and in the vicinity of the inner circumferential surface. Preferably, the thrust receiving member has a plurality of tapered apex portions and a central portion having a reduced diameter. In a preferred embodiment, an outer dimension of the bearing is rather larger than the rotary shaft insertion hole, and a circumscribed circle of the thrust receiving member is rather larger than the rotary shaft insertion hole. Each of said apex portions may be defined by a smooth curved surface. In an embodiment, said thrust receiving member is formed in a circular shape, and a plurality of communication holes for always communication said rotary shaft insertion hole and said through-hole with each other are formed in said thrust receiving member. Preferably, said communication holes and a circle of said rotary shaft insertion hole are crossed with each other for communication between the communication holes and said rotary shaft insertion hole at all times; and said through-hole is formed into an oblong hole which is always communicated with at least one of said communication holes. If the rotary shaft insertion hole is small so that a circle of the insertion hole is not crossed with the communication holes, a bevelled portion may be formed in the bearing with a bevel circle of the bevelled portion being crossed with the communication holes, thereby always communicating the rotary shaft insertion hole with the communication holes with each other through the bevelled portion; and the through-hole is in the form of an oblong hole so that the through-hole is always in fluid communication with at least one of the communication holes. Preferably, each of said communication holes is in the form of a circular hole or an oblong hole extending in the circumferential direction. Preferably, a bevelled portion is formed in an inner circumferential edge of the rotary shaft insertion hole on a side of the thrust receiving member, thereby always communicating the rotary shaft insertion hole with the through-hole with each other through the bevelled portion. The thrust receiving member may have an outer dimension which is equal to or somewhat smaller than a diameter of an inner circumferential surface of the bottomed recess and is laid on the bottom surface without any pressing forces; and the bearing is inserted under pressure into the bottomed recess, and the thrust receiving member sandwiched between the bearing and the bottom surface is kept under a non-rotatable condition. In an embodiment, said through-hole comprises at least one through-hole. Preferably, said thrust bearing member is formed of synthetic resin having a good lubrication. In an embodiment, said other bearing unit and said bearing are made of one selected from the group consisting of powder sintered metal and powder sintered alloy of iron copper system impregnated with lubricating oil. Additionally and/or alternatively, said other bearing unit and said bearing are made of one selected from the group consisting of plastics and general material. The casing may include: a housing formed of conductive material of a cold pressed steel plate in a bottomed hollow shape; and a cover member composed of a disk plate made of the same material as that of the housing and mounted on an opening portion of the housing. The one bearing unit and the other bering unit may be mounted on the cover member and the housing, respectively, a bottomed hollow cylindrical projection extending to an outside of the motor being integrally formed a central portion of the cover member, an inside of the projection is the bottomed recess, and an entirety of the bottom surface of the bottomed recess being made flat without any grooves. Said motor may be one selected from a miniature DC motor, an AC motor, and a stepping motor. In an embodiment, the thrust receiving member is made of synthetic resin having a good lubrication substantially in the form of a regular triangular with each apex portion being defined by a smooth curved surface, and the thrust receiving member having an outer dimension which is the same or somewhat smaller than a diameter of an inner circumferential surface of the bottomed recess is laid on the bottom surface without any pressing forces; the bearing is securely fixed within the bottomed recess, the thrust receiving member sandwiched between the bearing and the bottom surface being kept under a non-rotatable condition; and a circle of the rotary shaft insertion hole is smaller than an inscribed circle of the thrust receiving member, a bevelled portion is formed at an inner circumferential edge of the rotary shaft insertion hole on a side of the thrust receiving member, a bevel circle of a larger diameter portion of the bevelled portion is larger than the inscribed circle, and the rotary shaft insertion hole and the through-hole are always kept in fluid communication with each other through a space between the thrust receiving member and the inner circumferential surface. Thus, the thrust receiving member may be mounted within the bottomed recess without any pressing forces, and a gap between the bearing and the thrust receiving member may be dispensed with. Also, when the rotary shaft is rotated, the thrust receiving member is kept under a stationary condition and is not rotated. Accordingly, it is possible to shorten an axial length of the motor. Also, without deforming the thrust receiving member, it is possible to stably support the rotary shaft in the thrust direction. Also, the rotary shaft insertion hole formed in the bearing impregnated with the lubricating oil is in fluid communication with the through-hole formed in the bottom surface of the bottomed recess. Therefore, when the rotary shaft is inserted into the rotary shaft insertion hole formed in the bearing, the air that has been stagnant within the insertion hole is discharged to the outside of the motor through the through-hole. Accordingly, there is no risk that the air would be introduced into the interior of the bearing. Thus, there is no risk that the oil would splash to the motor interior by the air. The oil is prevented from sticking to a commutator, brushes or the like to thereby enhance the quality of the motor. Preferred embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a front sectional view showing a miniature electric motor of a first embodiment of the invention; Figure 2 is a perspective view showing an outer appearance of the motor shown in Figure 1; Figure 3 is an enlarged sectional view showing a primary part of the motor shown in Figure 1; Fig. 4 is a cross-sectional view taken along the line IV-IV of Fig. 3; Fig. 5 is a cross-sectional view showing a state where a thrust receiving member is set at a different position; Fig. 6 is an enlarged sectional view, similar to Fig. 2, showing a primary part of a motor according to a second embodiment of the invention; Fig. 7 is a cross-sectional view, similar to Fig. 4, taken along the line VII-VII of Fig. 6; Fig. 8 is an enlarged sectional view, similar to Fig. 3, showing a primary part of a motor according to a third embodiment of the invention; Fig. 9 is a cross-sectional view, similar to Fig. 4, taken along the line IX-IX of Fig. 8; Fig. 10 is a sectional view, similar to Fig. 4, showing a modification of the motor shown in Fig. 9; Fig. 11 is a sectional view, similar to Fig. 5, showing a part of a motor according to a fourth embodiment of the invention; Fig. 12 is a sectional view showing a modification of the motor shown in Fig. 11; Fig. 13 is a sectional view, similar to Fig. 4, showing a primary part of a motor according to a fifth embodiment of the invention; Fig. 14 is a sectional view showing a modification of the motor shown in Fig. 13; Fig. 15 is a sectional view showing another modification of the motor shown in Fig. 13; Fig. 16 is a histogram showing experimental results of change amounts of the rotary shaft of the motor in the radial direction according to the invention; Fig. 17 is a histogram showing experimental results of change amounts of the rotary shaft of the conventional motor in the radial direction; Fig. 18 is a front sectional view showing a conventional motor; Fig. 19 is a perspective view showing a bearing of the motor shown in Fig. 18; and Fig. 20 is a perspective view showing a bearing provided in another conventional motor. DESCRIPTION OF THE PREFERRED EMBODIMENTS The conventional structure will now be described with reference to Figs. 18 through 20 for the sake of better understanding the present invention. As shown in Figs. 18 and 19, in a miniature electric motor, a stator 2 is mounted on an inner surface of a casing 1, and a rotary shaft 6 of a rotor 5 is rotatably supported by bearing units 3 and 4 provided at both end portions of the motor. A recess 8 is formed in a cover member 7 of the casing 1, an arcuate groove 9 is formed in a peripheral portion of a bottom of the recess 8. An air vent hole 10 is formed in the groove 9. Through-holes 13 are formed in a bearing 11 for supporting the rotary shaft 6 in a radial direction in addition to a rotary shaft insertion hole 12, and are formed in parallel with an axial direction. A plate-like thrust bearing member 14 for supporting the rotary shaft 6 in a thrust direction is inserted under pressure into an interior of the recess 8. A predetermined distance h is kept between the bearing 11 and the thrust bearing member 14. For this reason, it is necessary to mold the arcuate groove 9 in the recess 8 and its molding work is difficult. In particular, in the case where the cover member 7 is made of metal, molds for press-molding the arcuate groove 9 would be remarkably damaged. Thus, the molds are inferior in durability and it is necessary to frequently carry out the maintenance. The thrust bearing member 14 is inserted under pressure into the recess 8. Therefore, if it is made of synthetic resin in the form of a plate with a good lubricant property, a warp would be likely to be formed in its peripheral portion in the case where the thickness of the plate is equal to or less than 0.25mm. Also in the case where the warp would not occur, its central portion is projected to lift away from the bottom of the recess 8. If the thrust bearing member 14 is deformed in this manner, a gap is generated between the bottom of the recess 8 and the thrust bearing member 14. Then, the rotor 5 is moved in the axial direction in the motor drive operation so that the contact between a commutator 15 and brushes 16 would be unstable, thus generating change in rotation. In order to prevent the above-described deformation of the thrust bearing member 14, it is proposed to set the thrust bearing member 14 on the bottom of the recess 8 by decreasing a diameter of the thrust bearing member 14 without any pressure. However, in this case, when the rotary shaft 6 is rotated, the thrust bearing member 14 is rotated together therewith. The service life of the thrust bearing member would be shortened, resulting from the friction between the thrust bearing member 14 and the bottom of the recess 8. On the other hand, if the bearing 11 would be in contact with the thrust bearing member 14 in order to shorten the overall length L of the motor in the axial direction, the rotary shaft insertion hole 12 is clogged with the thrust bearing member 14. Then, when the rotary shaft 6 is inserted into the rotary shaft insertion hole 12 in assembling the motor, an air stagnant in the insertion hole 12 would pass through the interior of the bearing 11. As a result, the lubricating oil impregnated in the bearing 11 would be splashed in the interior of the motor from the surface 17 of the bearing 11 together with the air. The splashed oil would be adhered to the commutator 15 and the brushes 16, resulting in failure of the electric contact. Also, the frictional worn powder would be adhered and accumulated thereto to cause the electric conduction between the commutator elements. Accordingly, it is necessary to form the gap h between the bearing 11 and the thrust bearing member 14 in the conventional structure. In addition, in the conventional structure, since the arcuate groove 9 is formed, a projection length of a projecting portion 18 where the recess 8 is formed is large. As a result, the overall length L of the casing 1 in the axial direction is elongated to make it difficult to thin the motor. By the way, in the case where the bearing 11 is impregnated with the lubricating oil as described above, a so-called "pump action" in which the oil in the bearing interior is moved to the surface of a space formed in the bearing interior and permeated onto the surface is generated. As a result of the pump action, the sliding part between the rotary shaft 6 and the rotary shaft insertion hole 12 is always lubricated. However, if the through-holes 13 are formed in addition to the rotary shaft insertion hole 12 as in this motor, or otherwise if cutaways 19 or the like for ventilating the air in the assembling the rotary shaft in the conventional bearing 11a as shown in Fig. 20 is formed, the pump action works outwardly from the center of the bearing 11 or 11a so that the oil is moved toward the surface of the through-holes 13 and the cutaways 19. As a result, the oil is insufficiently supplied to the rotary shaft insertion hole 12 to develop the wear of the sliding portions. For this reason, it is preferable not to form the through-holes 13 and the cutaways 19 in the bearing 11 or 11a. The present invention will now be described by way of example with reference to Figs. 1 through 17. (First Embodiment) Figs. 1 through 5 show a first embodiment of the invention. As shown in Figs. 1 and 2, a miniature DC motor 30 is provided with a casing 31. The casing 31 has a housing 33 in which a stator 32 is mounted, and a cover member 20 mounted on an opening portion 21 of the housing 33. A rotor 34 is provided within the casing 31. The housing 33 is made of conductive material such as a cold pressed steel plate mainly consisting of, for example, soft steel in the form of a bottomed hollow shape. Also, the cover member 20 engaged with the opening portion 21 of the housing 33 is formed of the same material in the form of a disk. Bearing units 36 and 37 mounted on both end portions of the casing 31 are to rotatably support a rotary shaft 35 of the rotor 34. One bearing unit 37 is mounted on the cover member 20 and the other bearing unit 36 is mounted on the housing 33. A bottomed hollow-shaped projection 38 projecting to an outside of the motor is integrally formed in a central portion of the cover member 20 by press-molding or drawing work. A bottomed recess 39 in which the bearing unit 37 is received is formed in an interior of the projection 38. A conventional arcuate groove is not formed in a bottom 54 of the bottomed recess 39 but the entire bottom 54 is made flat. The stator 32 is fixed to a cylindrical inner surface 40 of the housing 33 and is composed of a pair of permanent magnets each formed into an arc segment and made of magnetic material such as hard ferrite. The rotor 34 is provided with the rotary shaft 35 extending in a direction of a center axis C about which the rotor is rotated, an armature 43 which is mounted to the rotary shaft 35 and to which the rotational torque is imparted from the permanent magnet 32, and a commutator 44 assembled in a cylinder. The commutator 44 is mounted on the rotary shaft 35 and is electrically connected to the armature 43. An armature winding 41 is mounted in shape of coil around a rotor core 42 of the armature 43. The armature 43 is provided within the permanent magnet 32 at a predetermined air-gap relative to the permanent magnet 32. An oil stop washer 45 is securely fixed to the rotary shaft 35 at a lower end portion of the commutator 44 for preventing the oil of the bearing unit 37 from flowing toward the commutator 44. A plurality of (for example, two) pairs of brushes 46 each made of conductive material such as metal or carbon are provided in the cover member 20 so that the brushes 46 slidingly engage with the commutator 44. A plurality of (for example, two) connecting terminals 47 each electrically connected to the brush 46 are mounted on the cover member 20 through a brush holder 48 made of electrical insulating material such as synthetic resin or the like. A cylindrical projection 49 is formed in the central portion of the housing 33 by press-molding or drawing. The bearing unit 36 is fixed under pressure to an inner circumferential surface of the projection 49. The bearing unit 36 is to rotatably support the rotary shaft 35 on the output portion 50 side and to rotatably support the rotary shaft 35 in a radial direction. The bearing unit 37 serving to rotatably support an opposite output portion of the rotary shaft 35 is provided with a thrust receiving plate-like member 51 for supporting an end portion 57 of the rotary shaft 35 in a thrust direction, and a bearing 53 for supporting the rotary shaft 35 within a rotary shaft insertion hole 52 in the radial direction. It is possible to make the bearing unit 36 and the bearing 53 of plastic or general metal material. However, it is preferable to form these components 36 and 53 of powder sintered metal or powder sintered alloy of iron copper system impregnated with lubricating oil because the lubrication with the rotary shaft 35 is good and these components 36 and 53 are maintenance-free. It is preferable to form the thrust receiving member 51 of synthetic resin having a good lubrication. For example, it is possible to select, for this, Polyslider (trade name of K.K. Asahi Polyslider, in which additives are added to nylon 6) or Somasheet (trade name of Somar Corporation, in which additives are added to nylon 66). As shown in Figs. 1 and 3, the thrust receiving member 51 has a diameter that is equal to or smaller than the inner diameter of the bottomed recess 39 formed in the casing 31, i.e., equal to or somewhat smaller than the inner diameter. The thrust bearing member 51 is disposed on a planar bottom surface 54 of the bottomed recess 39. The bearing 53 is securely fixed in the bottomed recess 39 by pressing or the like, and is brought into pressing contact with the thrust bearing member 51. Thus, the thrust bearing member 51 is closely clamped or sandwiched between the bottom 54 and the bearing 53 so as not to be rotatable. A through-hole 56 that is in fluid communication with the outside of the motor is formed in the bottom plate 55 of the bottomed recess 39. The rotary shaft insertion hole 52 and the through-hole 56 are in fluid communication with each other. Namely, the thrust receiving member 51 has a predetermined shape not to cover an entire surface of the bottom surface 54 but to cover a part thereof so that a communication passage may be always maintained between the rotary shaft insertion hole 52 and the through-hole 56. In the embodiment shown, one single through-hole 56 is used but it is apparent that a plurality of through-holes may be used. In the motor 30 having the above-described structure, when a current flows from the connecting terminals 47 through the brushes 46 and the commutator 44 to the armature winding 41, a rotational torque is imparted to the rotor 34 that is present in the magnetic field formed by the pair of permanent magnets 32. Therefore, the rotor 34 takes a rotational motion. Then, the motor 30 drives a CD player (not shown) or the like through the output portion 50 of the rotating rotary shaft 35. The bearing unit 37 mounted on the bottomed recess 39 will now be explained in more detail. As shown in Figs. 3 and 4, it is preferable that the thrust receiving member 51 is formed substantially in a form of a polygonal shape, and that the rotary shaft insertion hole 52 and the through-hole 56 are always in fluid communication with each other through a space 61 between the thrust receiving member 51 and an inner circumferential surface 60 of the bottomed recess 39. Accordingly, in the embodiment shown, the thrust receiving member 51 is formed substantially in a regular triangle. The thrust receiving member 51 is laid on the bottom surface 54 of the bottomed recess 39 without pressing forces, whereby it is possible to avoid any deformation such as projection of a central portion or warp of a peripheral portion of the thrust receiving member 51. For this purpose, an outer dimension of the thrust receiving member 51 (for example, the diameter of a circumscribed circle 62 where all the apex of the polygonal shape are located on the circumference) is equal to or somewhat smaller than the diameter of the inner circumferential surface 60 of the bottomed recess 39. It is also preferable that the outer diameter of the thrust receiving member 51 is close to the diameter of the inner circumferential surface 60 as shown in Fig. 4 because the thrust receiving member 51 is hardly displaced in the radial direction within the bottomed recess 39 and may be settled in place. It is also preferable that each apex 58 of the through receiving member 51 is defined in a smooth curve because the apex 58 would not be hooked by the bottomed recess 39 and the bearing 53. If a circle of the rotary shaft insertion hole 52 would be smaller than an inscribed circle 65 of the thrust receiving member 51 which circle is located within the polygonal shape and is in contact with each side of the polygonal shape, the thrust receiving member 51 would clog the rotary shaft insertion hole 52. In view of the above, in the embodiment shown, a bevelled portion 63 is formed by bevelling an inner circumferential edge of the rotary shaft insertion hole 52 on the side of the thrust receiving member 51, and a bevel circle 64 of the larger diameter portion of the bevelled portion 63 is set to be larger than the inscribed circle 65 of the thrust receiving member 51. Thus, it is possible to always keep the fluid communication between the rotary shaft insertion hole 52 and the space 61. Fig. 5 shows the case where the thrust receiving member 51 is located at a different position from that shown in Fig.4. In this case, even though one of the apexes 58 of the thrust receiving member 51 extends towards the through-hole 56, the through-hole 56 is not closed by the thrust receiving member 51. Assuming that a center of the circle of the through-hole 56 is represented by b, the following relation is given: f > e where e is the dimension of the thrust receiving member 51 extending along a diameter of the through-hole 56 in a direction perpendicular to a line d passing through the center b and the center of the thrust receiving member 51, and f is the diameter of the through-hole 56. To meet this relation,it is possible to always bring the space 61 and the through-hole 56 into fluid communication with each other whenever the thrust receiving member 51 is laid on the bottom surface 54 of the bottomed recess 39 about the center axis C. Accordingly, there is no limit to mounting the thrust receiving member 51 and the latter 51 may be set without pressing forces. It is therefore easy to assemble the unit to thereby avoid the assembling error. In assembling the motor, when the rotary shaft 35 is inserted into the rotary shaft insertion hole 52, the air which is stagnant within the insertion hole 52 will flow to the space 61 through the bevelled portion 63. The air is discharged from the space 61 through the through-hole 56 to the outside of the motor 30. Accordingly, since the air is not caused to pass through the interior of the bearing 53, there is no fear that the lubricating oil impregnated in the interior of the bearing 53 would be splashed into the interior of the motor together with the air and would be adhered to the sliding parts or the like between the commutator 44 and the brushes 46, which is encountered in the conventional motor. Incidentally, in the case where the oil comes out to the side of the space 61 from the bearing 53, the oil is to be discharged to the outside of the motor and does not splash to the interior of the motor. (Second Embodiment) Figs. 6 and 7 show a second embodiment of the invention. In the second embodiment, the diameter of the rotary shaft insertion hole 52a formed in a bearing 53a is large. Namely, the circle of the rotary shaft insertion hole 52a is larger than the inscribed circle 65 of the substantially regular triangular thrust receiving member 51. In this case, even if the bevel of the bearing as shown in the first embodiment is not effected, the rotary shaft insertion hole 52a and the space 61 are always in fluid communication with each other. Accordingly, since it is unnecessary to form the bevelled portion in the bearing 53a, it is possible to take a large sliding area between a rotary shaft 35a and the bearing 53a. The other structure of the second embodiment is the same as that of the first embodiment and the same effect is insured. (Third Embodiment) Figs. 8 and 9 show a third embodiment of the invention. In the third embodiment, a substantially regular hexagonal thrust receiving member 51b is used as the polygonal thrust receiving member. A circumscribed circle 62b of the thrust receiving member 51b is substantially equal or somewhat smaller than the inner circumferential surface 60. The thrust receiving member 51b is laid within the bottomed recess 39 without pressing forces. In the case where this thrust receiving member 51b is used, the space 61 is smaller in radial direction than that of the foregoing triangular thrust receiving member 51 and is far away from the rotary shaft insertion hole 52. For this reason, in this embodiment, a bearing 53b is largely bevelled to cause a bevelled portion 53b to be in fluid communication with the space 61. More specifically, a bevel circle 64b of the bevelled portion 63b is larger than an inscribed circle 65b of the thrust receiving member 51b. Therefore, the rotary shaft insertion hole 52 is always kept in fluid communication with the space 61 through the bevelled portion 63b. If a slant angle  of the bevelled portion 63b relative to a surface 59 of the thrust receiving member 51b is kept small, it is possible to ensure a large sliding area between the bearing 53b and the rotary shaft 35. Also, in order to always bring the space 61 and the through-hole 56 in fluid communication with each other, the through-hole 56 is formed at the position in the vicinity of the inner circumferential surface 60. Incidentally, the other structure of the third embodiment is the same as that of the first embodiment to ensure the same advantage. Fig. 10 is a view showing a modification of the embodiment shown in Fig. 9. A thrust receiving member 51c is shaped to a substantially polygonal shape such as a shape similar to a regular square. Namely, an arcuate portion 67 having a center concentric with the inner circumferential surface 60 is formed in each apex 66 of the thrust receiving member 51c. In this case, it is easy to manufacture the thrust receiving member 51c by cutting peripheral portions of a disk plate along a straight line. It is preferable to form a through-hole 56c into an oblong hole (for example, a hole extending in the circumferential direction) in order to always communicate with the space 61, and to locate it in the vicinity of the circumferential surface 60. Incidentally, the other structure of this modification is the same as that of the third embodiment shown in Fig. 9. (Fourth Embodiment) Figs. 11 and 12 show a fourth embodiment of the invention. As shown in Figs. 11 and 12, a thrust receiving member 51d according to this embodiment has a plurality of (for example, three) tapered apex portions 70 and a reduced diameter central portion 72. The rotary shaft insertion hole 52 and the through-hole 56 are always kept under the fluid communication with each other through the space 61 between the thrust receiving member 51d and the inner circumferential surface 60 of the bottomed recess 39. The thrust receiving member 51d having such a structure is suitable for the case where the outer dimension of the bearing 53 is much larger than that of the rotary shaft insertion hole 52. Namely, since the diameter of a circumscribed circle 62d of the thrust receiving member 51d is rather large, if the thrust receiving member is in the form of a polygonal shape such as a regular triangle, the rotary shaft insertion hole 52 would be clogged or closed. Accordingly, as shown in Fig. 11, if the central portion 72 is reduced in diameter and the inscribed circle 65d is smaller than the circle of the rotary shaft insertion hole 52, it is possible to always maintain the fluid communication between the rotary shaft insertion hole 52 and the space 61. In the case where the circle of the rotary shaft insertion hole 52 is smaller than the inscribed circle 65d of the thrust receiving member 51d as shown in Fig. 12, the bevelled portion having the bevel circle 64d that is larger than the inscribed circle 65d in the same manner described above may be formed in the bearing. Additionally, if each apex portions 70 are defined by smooth curved surfaces, each apex portions 70 would not be hooked by the bottomed recess 39 or the bearing 53. Also, according to this embodiment, it is possible to reduce a weight of the motor because a weight of the thrust bearing member 51d is small. Incidentally, the other structure of the fourth embodiment is the same as that of the first embodiment to ensure the same advantage. (Fifth Embodiment) Figs. 13 through 15 show a fifth embodiment of the invention. In this embodiment, the thrust receiving member is formed into a circular shape, and a plurality of communication holes 71 and 71e are formed in the thrust receiving member so as to always communicate with the rotary shaft insertion hole 52 and a through-hole 56e formed in the bottom plate 55 of the bottomed recess 39. Fig. 13 shows a case where no bevel portion is formed in the bearing. The plurality of communication holes 71 are formed in a circular thrust receiving member 51e, and the communication holes 71 are arranged uniformly in the circumferential direction. Also, by crossing the communication holes 71 with the circle of the rotary shaft insertion hole 52, the communication holes 71 are always kept in fluid communication with the rotary shaft insertion hole 52. In order to always communicate the through-hole 56e with at least one of the communication holes 71, the through-hole 56e is formed in an oblong hole (for example, a hole extending in the circumferential direction). Fig. 14 shows a thrust receiving member 51f where the bevelled portion is formed in the bearing to compensate for the smaller diameter of the rotary shaft insertion hole 52. The circle of the rotary shaft insertion hole 52 does not cross the communication hole 71 but a bevel circle 64e of the bevelled portion does across the communication holes 71. Accordingly, the rotary shaft insertion hole 52 and the communication hole 71 are always kept in fluid communication with each other through the bevelled portion. In addition, the communication holes are not limited to the circular ones but may be formed into a plurality communication holes 71e like oblong holes (for example, holes extending in the circumferential direction) formed in a thrust receiving member 51g as shown in Fig. 15. (Experimental Results) Fig. 16 shows experimental results of change amount (i.e., displacement amount) in the radial direction of the rotary shaft 35 of the miniature motor 30 according to the present invention. Fig. 17 shows experimental results of change amount (i.e., displacement amount) in the radial direction of the rotary shaft 6 of the conventional miniature motor shown in Fig. 18. In each graph, the axis of ordinate represents the change amount and the axis of abscissa represents the distribution of the number of the motors. According to this experiment, forty motors were driven and measured for a long period of time (1,200 hours) with being coupled with a compact disk player for each case of the present invention and the conventional system. In the motor 30 according to the invention, since the holes or cutaways other than the rotary shaft insertion hole 52 were not formed in the bearing 53, the lubricating oil impregnated in the bearing 53 was moved toward the rotary shaft insertion hole 52 by the pumping action and hence was supplied sufficiently to the sliding parts between the bearing 53 and the rotary shaft 35. Accordingly, as shown in Fig. 16, the change amount of the rotary shaft 35 was very small. In contrast, in the conventional motor as shown in Fig. 18, since the through-holes 13 are independently formed in the bearing 11, the oil impregnated within the bearing 11 was moved toward the through-holes 13 by the pumping action, resulting in insufficient supply to the sliding parts between the bearing 11 and the rotary shaft 6. Accordingly, the lubrication was insufficient and the wear of the sliding parts of the bearing 11 was remarkable, resulting in the increased change amount as shown in Fig. 17. As described above, according to the invention, the arcuate groove that is required in the conventional motor may be dispensed with. Since no gap is formed between the bearing 53 and the thrust receiving member of the motor 30, it is possible to shorten the overall axial length L 1 of the motor (see Fig. 1) to thereby thin the motor 30. Also, since the thrust receiving member is set within the bottomed recess 39 without any pressing forces, there is no fear that the thrust receiving member would be deformed, and it is possible to stably support the rotary shaft 35 in the thrust direction. Accordingly, the rotary shaft 35 is not moved in the axial direction, and the commutator 44 and the brushes 46 are brought into stable contact with each other, thus rotating the motor at a constant rotational speed. Since the thrust receiving member may be mounted within the bottomed recess 39 in any direction, it is easy to assemble the motor and to avoid the mounting error. The thrust receiving member is set within the bottomed recess 39 without any pressing forces but is securely fixed between the bearing and the flat bottom surface. As a result, the thrust receiving member never rotates when the rotary shaft is rotated. Accordingly, there is no fear that the thrust receiving member would be worn due to the friction between the thrust receiving member and the bottom surface. When the rotary shaft is inserted into the rotary shaft insertion hole of the bearing impregnated with the lubricating oil, the air within the rotary shaft insertion hole is discharged from the space to the outside of the motor through the through-hole. Accordingly, the air will not flow through the bearing. There is no fear that the oil impregnated within the bearing would splash into the interior of the motor by the air. The oil is prevented from sticking to the commutator or the brushes to stabilize the contact. Thus, there is no fear of the reduction in performance and the adverse affect to the service life. Therefore, the quality of the motor is enhanced. Also, according to the invention, any holes or cutaways other than the rotary shaft insertion hole are not formed in the bearing of the bearing unit 37. Accordingly, the oil is sufficiently moved toward the rotary shaft insertion hole by the pumping action, and the rotary shaft 35 may be smoothly rotated within the rotary shaft insertion hole with a reduced wear. Also, the overall bottom surface 54 where the conventional arcuate groove is not formed is made flat. It is therefore to facilitate the molding when the cover member 20 is formed by press molding. Also, the damage of the molds for molding may be suppressed and the durability thereof is enhanced as described below. The dimension of the molds may be kept under a high precision condition for a long period of time. (Experimental Results of Durability of Molds) Molds for cover member 20 according to the invention About ten millions of shots Molds for the conventional cover member 7 About five millions of shots In the foregoing embodiments, the invention is applied to the case where the bearing unit 37 is mounted on the cover member 20 but it is apparent that the invention may be applied to the case where the bottomed recess is formed in the housing 33 and the bearing unit 37 is mounted therein. Preferably, the invention is applied to a thin miniature motor, and it is possible to take a posture of the motor in use either in a vertical position or a horizontal position. Although the invention may be allied to a miniature DC motor as described above, it is apparent that the invention may be applied to any other type of miniature electric motor such as an AC motor, a stepping motor or the like as desired. In the drawings, the same reference numerals are used to indicate like components or members. It will be appreciated that modifications and variations may be made to the embodiments described and illustrated within the scope of the appended claims.

Claims: A miniature electric motor (30) in which a rotor (34) is arranged in a casing (31) within which a stator (32) is mounted, a rotary shaft (35) of said rotor (34) being rotatably supported by spaced bearing units (36,37) mounted on said casing (31), wherein at least one (37) of said bearing units comprises: a thrust receiving member (51, 51b to 51g) for supporting an end portion (57) of said rotary shaft (35, 35a) in a thrust direction, said thrust receiving member being laid on a substantially planar base surface (54) of a recess (39) formed in the casing (31), and a bearing (53, 53a, 53b) securely fixed in said recess (39) and in pressing contact with said thrust receiving member (51, 51b to 51g), said bearing having a rotary shaft insertion hole (52, 52a) in which said rotary shaft (35, 35a) is received; characterised in that said rotary shaft insertion hole (52, 52a) is arranged to be in fluid communication with the outside of the casing (31) by way of a through hole (56, 56c, 56e) formed through a base (55) of said recess (39), and in that said rotary shaft insertion hole and said through hole are maintained in fluid communication around and/or through said thrust receiving member (51, 51b to 51g), said thrust receiving member having a size and peripheral shape to enable said fluid communication. A motor as claimed in Claim 1, wherein said thrust receiving member (51, 51b to 51g) is in the form of a plate and has a size and peripheral shape such that it does not cover the substantially planar base surface (54) on which it is laid whereby said through-hole (56, 56c, 56e) and said rotary shaft insertion hole (52, 52a) are maintained in fluid communication by way of a spacing (61) thereby defined between the periphery of said thrust receiving member and an inner surface (60) of said recess. A motor as claimed in Claim 1 or Claim 2, wherein said through-hole (56) is circular and has a centre b, and wherein the following relationship is given: f > e where e is the dimension of the thrust receiving member (51, 51b to 51g) extending along a diameter of the through-hole (56) and in a direction perpendicular to a line d passing through the centre b and the centre of the thrust receiving member, and f is the diameter of the through-hole (56). A motor as claimed in any of Claims 1 to 3, wherein a bevelled portion (63, 63b) extends from an inner circumferential edge of said rotary shaft insertion hole (52) facing the thrust receiving member, a bevel circle (64, 64b, 64d) of said bevelled portion being larger than an inscribed circle (65, 65b, 65d) of said thrust receiving member, and said inscribed circle (65, 65b, 65d) of said thrust receiving member having a larger diameter than that of said inner circumferential edge of said rotary shaft insertion hole (52) whereby fluid communication between said rotary shaft insertion hole (52) and said through-hole (56) is maintained. A motor as claimed in any preceding claim, wherein a plurality of communication holes (71, 71e) for communicating said rotary shaft insertion hole (52) and said through-hole (56) are formed in said thrust receiving member (51, 51b to 51g). A motor as claimed in any preceding claim, wherein: said thrust receiving member (51, 51b to 51g) has an outer dimension which is equal to or somewhat smaller than a diameter of an inner circumferential surface (60) of said recess and is laid on said base surface (54) without any pressing forces; wherein said bearing (53, 53a, 53b) is inserted under pressure into said recess, and wherein said thrust receiving member is sandwiched between said bearing and said base surface (54) and is kept thereby in a non-rotatable condition. A motor as claimed in any preceding claim, wherein said thrust receiving member (51, 51b to 51g) is formed of synthetic resin having a good lubrication. A motor as claimed in any preceding claim, wherein said bearings (36,53, 53a, 53b) are made of a material selected from a group comprising powder sintered metal and powder sintered alloy of iron copper system impregnated with lubricating oil. A motor as claimed in any preceding claim, wherein said casing (31) comprises a housing (33) formed of conductive material and substantially cup-shaped, and a cover member (20) made of the same material as that of the housing (33) and mounted on an opening (21) of said housing; wherein said one bearing unit (37) is mounted on said cover member (20) and a spaced bearing unit (36) is mounted on said housing, and wherein a closed hollow cylindrical projection (38) extending externally of the casing is integrally formed in a central portion of said cover member (20), the inside of said projection (38) defining said recess (39), the entirety of said base surface (54) of said recess (39) being substantially flat and without any grooves. A motor as claimed in any preceding claim, wherein said motor is one selected from a miniature DC motor (30), an AC motor and a stepping motor. A motor as claimed in any preceding claim, wherein: said thrust receiving member (51) is made substantially in the form of a regular triangle with each apex portion (58) being defined by a smooth curved surface, said thrust receiving member having an outer dimension which is the same or somewhat smaller than a diameter of an inner circumferential surface (60) of said recess (39) and being laid on said base surface (54) of said recess without any pressing forces; wherein said bearing (53) is securely fixed within said recess such that said thrust receiving member is sandwiched between said bearing and said base surface (54) whereby it is kept in a non-rotatable condition; and wherein a circle of said rotary shaft insertion hole (52) is smaller than an inscribed circle (65) of said thrust receiving member, a bevelled portion (63) is formed at an inner circumferential edge of said rotary shaft insertion hole facing said thrust receiving member, a bevel circle (64) of a larger diameter portion of said bevelled portion is larger than said inscribed circle, and said rotary shaft insertion hole and said through-hole (56) are always kept in fluid communication with each other through a space (61) between said thrust receiving member and said inner circumferential surface (60).

Inventors: HARANO JUNICHI, IWAMOTO SHIGEYOSHI, KASE MITSUO, HARANO, JUNICHI, IWAMOTO, SHIGEYOSHI, KASE, MITSUO, HARANO, JUNICHI, MABUCHI MOTOR CO.,LTD., IWAMOTO, SHIGEYOSHI, MABUCHI MOTOR CO.,LTD., KASE, MITSUO, MABUCHI MOTOR CO.,LTD.
Applicants: MABUCHI MOTOR CO, MABUCHI MOTOR CO., LTD
Application year: 1994
Main classification: H02K5
IPC codes: H02K5, F16C35, F16C17